Dirichlet's Box Principle/Also known as

Dirichlet's Box Principle: Also known as

Dirichlet's Box Principle, in particular its corollary, is also commonly known as the pigeonhole principle or pigeon-hole principle:

Suppose you have $n + 1$ pigeons, but have only $n$ holes for them to stay in.

By the pigeonhole principle, at least one of the holes houses $2$ pigeons.

It is also known as (Dirichlet's) drawer (or shelf) principle.

Some sources give it as the letterbox principle or letter-box principle.

Sources

• 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): letter-box principle
• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): Dirichlet's principle
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): Dirichlet's principle